An Allied intelligence report on the effectiveness of spaced armor during WWII, from Tactical and Technical Trends, No. 39, December 2, 1943. [DISCLAIMER: The following text is taken from the U.S. War Department publication Tactical and Technical Trends. As with all wartime intelligence information, data may be incomplete or inaccurate. No attempt has been made to update or correct the text. Any views or opinions expressed do not necessarily represent those of the website.]

ENEMY USE OF SPACED ARMOR A recent British study provides a statement of the general principles governing the attack of, and defense by, spaced armor. The essential points bearing on the use of spaced armor by the enemy, that follow, are taken from the British report. *         *         * Two separated homogeneous plates attacked by A.P. shot, or 2 separate plates in loose contact, are less effective in defense than a single plate of the same total thickness. This is because there is no shearing resistance over the internal free surfaces. A front cemented (face-hardened) plate separated from a rear homogeneous plate attacked by A.P.C. (armor-piercing capped) shot will, for the same reason, offer less resistance than a single cemented plate of the same total thickness. A front homogeneous plate separated from a rear cemented plate offers increased resistance to A.P.C. shot as the front homogeneous plate destroys the cap of A.P.C. shot and so diminishes the effectiveness of the projectile against the cemented surface of the rear plate. The important point in this construction is that by the use of spaced plates the cemented face can be made to be an interior surface of the system, and so afford protection against an A.P.C. shot de-capped by the front plate. For the same reason a rear cemented plate offers increased protection against a tungsten carbide cored shot, unless the core is very well protected by a cap and some form of cushion. A rear cemented plate will also offer greatly increased protection against an A.P. shot because the nose of the A.P. shot will be partly damaged by the front homogeneous plate, and so will have a still further diminished power against the cemented surface of the internal plate. From the point of view of attack, the effectiveness of capped (A.P.C.) and uncapped (A.P.) projectiles varies in different calibers owing to a differing balance of advantages and disadvantages. Though a cap tends to preserve the point of the ogive (the curved and pointed head of the projectile) during perforation of the front plate, the cap itself is destroyed by the same action, and thus kinetic energy is wasted. In one caliber the reduction of energy due to loss of the cap before reaching the interior plate may result in failure to perforate, whereas in a larger caliber the loss may have a less pronounced effect on the ultimate performance, resulting in perforation of 2 plates of the same quality and equally well matched. It is true that an A.P. or A.P.C. shot tends to turn towards the normal on perforating a plate, but in the case of spaced armor any advantage which might be gained thereby is likely to be neutralized by the acquisition of a transverse angular velocity which may result in increased yaw. For this effect to be appreciable, plates should be separated by a distance of at least one caliber.

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